“Apple is under no legal obligation to do that and should never be forced to do that.”
No legal obligation??
What about a moral obligation to protect innocent American lives? Not everyone is like you, who would rather be killed by a terrorist than have Apple try to save lives.
No lives will be saved by cracking the terrorist's phone although the government would probably argue otherwise. Specifically the one or two suspects that might be pulled from that phone are swamped by the next 1000 "lone wolf" terrorists in this country waiting for their turn. The police need to do some more policing but that will have to start with "moderate" Muslims turning in the other Muslims. If that doesn't happen, then all the decryption in the world isn't going to help.
To force a safe maker to purposely break an uncrackable safe, even once, even for the best reasons (which they are not), is bad precedent. It means that any phone will be updatable to stop the key wiping. It means any company can be forced to do the same. Around the world there are plenty of countries that will demand the same and use the precedent here to demand it.
What most people are missing is what the government is obfuscating by choosing the phone of a dead perp. Ordering the unlocking of a phone can be done by tossing the owner in jail provided there is sufficient evidence of wrong doing to do so. If there is not sufficient evidence then the phone stays locked.